3.1.2 FlipFlopApplications.pptx

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Flip-Flop Applications

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This presentation will provide an overview of

the following flip-flop applications:

• Event Detect

• Data Synchronizer • Shift Register

• Frequency Divider

Additional flip-flop applications will be covered in future lessons:

• Asynchronous {Ripple} Counters • Synchronous {Parallel} Counters • State Machines

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Event Detector

• _{An event detector circuit is one that is capable of}

detecting and holding a signal until the event that changed the signal can be addressed.

• _{For example, door sensors on a home burglar alarm}

system use an event detector circuit. Once someone opens the door, the alarm will sound until the system is turned off.

• _{If it were not for the event detection circuit, the alarm}

would only be on when the door was open. Thus,

someone could quickly open the door, enter the house, and close the door. In this situation the alarm would only sound for the brief time that the door was actually open.

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Event Detect: Circuit

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Event Detect

Single_Event Clear

Held_Event

Single_Event Clear

Held_Event

Event Held

Event Cleared

Multiple Events

Only the 1st_{event is held.}

Multiple Clears

Only the 1st_{event does anything.}

Case #1

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Data Synchronizer

Normally, timing is not a concern with combinational logic.

However, due to gate delays, the outputs of combinational logic can change at different times. This can lead to potential timing problems. Take a look at the combinational logic circuit shown below. Because functions A, B, & C have different circuit

complexity, they change logic levels at different times.

To solve this problem, a data synchronizer circuit can be added to

the outputs. 6

C B A Combinational Logic Circuit . . . Inputs

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Data Synchronizer: Timing

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Latched_A Latched_B Latched_C A

B C Clock

The signal can be a zero or a one, depending on the previous latched values.

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Frequency Divider

• _{As the name implies, a frequency divider}

is a circuit the produces a digital output

signal that is half the frequency of the

input.

• _{The frequency divider is used extensively}

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Frequency Divider: Circuit

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Clock_Out

Clock_In

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Frequency Divider: Timing

11 Clock_Out Clock_In T₁ T₂

The period of

Clock_Out is twice the period of

Clock_In.

Since the period is twice, the frequency is divided in half.

frequency) the (half : F ƒ Clock_Out of Frequency : T ƒ Clock_In of Frequency : T ƒ period) the (twice : T T Clock_Out of Period : T Clock_In of Period : T 2 2 2 1 1 2 1 2 2 1 1 1 2 1     

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Shift Register

• _{A shift register is a group of flip-flops (typically 4}

or 8) that are arranged so that the values stored in the flip-flops are shifted from one flip-flop to the next for every clock.

• _{Shift registers are used extensively in logic}

circuits to control digital displays.

• _{A classic example is numbers being typed into a}

calculator. As the numbers are entered, the

digits shift to the left one position. This shifting is controlled by a shift register.

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Shift Register

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Data In _{Data Out}

Data Out _{Data In}

Data Out Data In

Serial In / Serial Out

Left-to-Right

Serial In / Serial Out

Right-to-Left

Serial In / Parallel Out Parallel In / Serial Out

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Shift Register: Circuit (D)

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Shift Register: Timing

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Clock Data_In D1

D2 D3 D4

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MSI Shift Register

74LS194 (4-bit Bi-Directional Universal Shift Register)

Parallel Inputs Serial Inputs Mode Control

Parallel Outputs &

Serial Outputs

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74LS194 Shift Register: Circuit

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74LS194 Shift Register: Timing

Clock X4 X3 X2 X1